Patent GB-892552-A (GB)
Title: Improvements in electrical selective signalling equipment
IPC: H04L27/20
Filed: 1958-07-18
Abstract: 892,552. Electric signalling. STANDARD TELEPHONES & CABLES Ltd. July 18, 1958 [July 19, 1957], No. 23116/58. Class 40(1). Electric selective signals transmitted in a sequence of signal intervals comprise elements at the beginning of each interval for synchronizing the transmitter and receiver and a further element within the interval to indicate data. In the form of Fig. 1 the signal intervals M consists of two halves a1 a2, the synchronizing element being a change of polarity at the beginning of the interval and the data element, where it exists, being a further change of polarity in the middle of the interval. Wave from b indicates a succession of " 0's," the reversal of polarity at the beginning of each interval producing a square wave of frequency F. Wave from c indicates a succession of " 1's," the additional reversal of polarity in the middle of each interval producing a wave of frequency 2F. If a series of binary signals 0100110 as shown at d is to be transmitted, the wave e is derived having changes at the beginning of each interval and in the middle of those intervals which correspond to binary " 1's." The code could be reversed, the binary " 0 " having a polarity change instead of binary " 1."  Transmitter, Fig. 3.-The time base BT produces pulse trains a and b which, as shown in Fig. 4a, 4b, are in antiphase and have a frequency 2F (two to each interval) monostable flip-flops BMa and BMb form from these pulse trains, trains of short pulses c and d at frequency F the former corresponding to the beginning of an interval and the latter to the middle. Pulses d pass to a circuit and serve to synchronize the generation of the sequence (say 0100110) to be sent. This sequence appears at e and is applied to a bistable flipflop BB to which the pulses c are also applied. The state of flip-flop BB is reversed by pulses c if the signal e is negative (corresponding to binary " 0 ") and a wave f, Fig. 4, is produced for application to a second bi-stable flip-flop BD. Pulse trains c and d also pass to this flipflop and are directed to one or other input depending upon the state of flip-flop BB. Pulses c and d can therefore switch flip-flop BD in either direction and complementary outputs g and h, Fig. 4, assume the desired waveform for transmission, e.g. by modulating a carrier frequency. Receiver, Fig. 5.-The signals received appear at a and b, Fig. 6. A decoding circuit CD forms from these signals a pulse train c, having a pulse for each polarity change, and a pulse train d having a pulse in the second half of each signal interval without a change of polarity; such pulses represent the binary " 0's." Pulse trains c and d are applied to flip-flop BB which starts each interval in one state and is changed to the other by the mid-interval c pulses for a binary " 1 " or by the d pulses for a binary " 0." The latter being somewhat later cause the flip-flop to stay in the second state for a shorter time in a " 0 " interval. The output e is passed to a synchronizing circuit CS which derives pulses f representing the start of each interval and pulses g representing the middle. Another bi-stable flip-flop BP controlled by pulses d produces an output h, Fig. 6, a change in which represents a " 0." A further flipflop BR is operated by pulses g under the control of waveform h. This flip-flop changes state at the middle of each signal interval (pulses g) if in the preceding interval flip-flop BP has changed, that is if the preceding digit was " 0." The output i, Fig. 6, therefore follows the form of wave h, the two waves being the same, both positive or both negative, when the preceding signal corresponded to " 1 " but are different when it was " 0." Flip-flop BS is changed to " 1 " state by g pulses if the i and h waveforms are the same and to " 0 " state if they differ. The output j, Fig. 6, with a time shift of one and a half intervals, represents the original data 0100110, " 1 " being represented by a positive signal " 0 " by a negative signal. These signals are applied to a scanning device where they are interpreted with the aid of the synchronizing pulses f and g. Transmitter and receive circuits using transistors are described in detail.